1. Field of the Invention
The present invention generally relates to semiconductor manufacturing process, and more particularly, to a method for separating photomask patterns.
2. Description of the Prior Art
Optical lithography is a crucial step in semiconductor manufacturing. The basic principle of optical lithography is quite similar to that of chemistry-based photography. Images of a patterned photo-mask are projected through high-precision optical systems onto a wafer surface, which is coated with a layer of light-sensitive chemical compound, e.g. photo-resist. The patterns are then formed on the wafer surface after complex chemical reactions and follow-on manufacturing steps, such as development, post-exposure bake, and wet or dry etching.
As the gap between the required feature size and the lithography wavelength gets bigger, the final wafer images are quite different from the original patterns of the mask. In the past few years, resolution enhancement techniques (RETs) have become necessary in order to achieve the required pattern density. One well-known RET is the optical proximity correction (OPC), in which the mask patterns are intentionally “distorted” so that the desired image can be formed on the wafer. Nowadays, considerable amount of computing power has to be dedicated to these kinds of post-layout processes.
In addition to the above-mentioned OPC technology, a process of double patterning has been developed as a cost-effective way to further scale into the deep submicron domain, using the same lithographic technology. One popular form of double patterning is the double exposure lithography, wherein a given layout is split or decomposed into two sets of patterns, each of which is printed using a separate mask in a separate exposure step. The desired layout may be constructed by these two separate patterns.
The greatest advantage of the double exposure lithography is that we can use available photo-lithography technology and tools to manufacture finer patterns with even higher density. However, there are still many process issues to overcome in practice. It is still necessary to improve the conventional pattern split method for double exposure process.